The system of toxicity equivalents (TEQ) was introduced as a means of enhancing assessment of the toxicity of dioxins and dioxin-like polychlorinated biphenyls (PCBs). The TEQ system makes it possible to express a range of different concentrations of substances that all have the same toxicological active profile, but different toxic effects, as a single, combined value. So-called "toxicity equivalent factors" (TEF) play a key role in calculation of TEQ.
Under the system, toxicity of individual substances is expressed in terms of 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD), which is considered to be the most toxic compound in the substance group and which achieved notoriety as the "Seveso dioxin". As the reference substance for the system, 2,3,7,8-TCDD has been assigned a factor of "one". Other compounds in the dioxins / dioxin-like PCBs substance group have received correspondingly lower values in proportion to their lower toxicity. In any given case, the different compounds involved, with different toxicity equivalent factors, and with the different quantities in which they are present, account for different shares of the pertinent TEQ. The toxicity equivalent factors are determined in the framework of scientific studies on the toxicity of the various relevant compounds.
Consequently, the factors have to be updated at regular intervals, taking account of any new findings. In 1998, the World Health Organization (WHO) compiled a list of toxicity equivalent factors. For some time thereafter, that list was used as a basis for defining the EU's maximum permitted levels for dioxins and dioxin-like PCBs in foods. In 2005, the toxicity equivalent factors were reviewed, and new values were proposed, based on evaluations of new studies using suitable mathematical and statistical techniques. With the new values, the toxicity of dioxins and dioxin-like PCBs in foods is classified as ten to 20 percent lower, depending on the food involved, than the corresponding values obtained with the toxicity equivalent factors from 1998.
The new maximum-level and action-level regulations applying since 1 January 2012 for dioxins and dioxin-like PCBs in foods are oriented to the toxicity equivalent factors from 2005.